US Vice President Kamala Harris has challenged Donald Trump to join her on stage for another presidential debate after she accepted an invitation from CNN.

The Democrat was widely believed to have won the last matchup in which Trump generated headlines with bizarre claims of pet-eating immigrants.

CNN is proposing a second debate on October 23, less than two weeks before election day on November 5.

Harris’s campaign said the vice president was “ready for another opportunity to share a stage with Donald Trump”.

“She has accepted CNN’s invitation to a debate on October 23.

“Donald Trump should have no problem agreeing to this debate.”

However former president Trump on Sunday (AEST) ruled out a second debate, and claimed Harris only wanted it because she was “losing badly”.

Trump said it was “too late” for another debate because Americans had already begun casting their ballots for the November 5 election.

The first in-person voting began in Virginia, Minnesota and South Dakota with several more states to follow by the middle of October.

“It’s too late to do another, I’d love to in many ways but it’s too late, the voting is cast, the voters are out there, immediately – is everybody voting, please? Get out and vote,” said Trump at a rally in Wilmington, North Carolina.

“She’s had her chance to do it with Fox. You know, Fox invited us on, and I waited and waited, and they turned it down.

“They turned it down, but now she wants to do a debate right before the election with CNN because she’s losing badly.”

During a stop in California last week, Trump seemed to leave a crack open for a possible second showdown, stating: “Maybe if I got in the right mood.”

CNN said it sent invitations to both candidates because “we believe the American people would benefit from a second debate between the two candidates for President of the United States”.

“We look forward to receiving a response from both campaigns so the American public can hear more from these candidates as they make their final decision.”

Harris and Trump debated each other for the first time on September 10 in Philadelphia, in a contest hosted by ABC that polls showed was won by the Democratic nominee.

Harris’s huge edge

Meanwhile, on Friday (US time) new filings revealed that Harris has a dramatic advantage over Trump when it comes to campaign monies.

The vice president had nearly quadruple the amount of money flowing into her campaign compared to Trump in August.

Harris scooped $US190 million ($279 million) to her principal campaign versus Trump’s $US44.5 million ($65 million).

Her campaign also outspent Trump’s in August — splurging $US174 million ($255 million) compared to Trump’s $US61 million ($89 million).

CNN reports that the Democrats are enjoying an edge over the Republicans with advertising spending in key battleground states and are overwhelming the former president’s presence on social media.

With 45 days before election day, a new poll has encouraging results for Harris.

The Guardian‘s 10-day polling averages survey showed Harris ahead of Trump 48.5 per cent to 45.9 per cent — a lead of 2.6 points and a 0.9 per cent improvement on last week.

A New York Times/Philadelphia Inquirer/Siena nationwide poll on Thursday showed them tied at 47 per cent.

Early voting begins

US voters have begun casting their first in-person ballots is a stage that marks a six-week stretch to the November 5 election day.

It means the end is now in sight to the bitterly divided campaign.

Most states offer some form of in-person early voting.

The first in-person voting was in Virginia, Minnesota and South Dakota with several more states to follow by the middle of October.

While Trump has frequently expressed scepticism about early voting in the past, the Republican National Committee has embraced the concept this year as an important way to pile up votes in advance, since weather and other factors can affect the turnout on November 5.

Democrats have taken advantage of early voting options in recent elections, banking millions of votes in the process.

In 2018, Democrats accounted for 41 per cent of early votes in the 24 states that provide partisan data, compared to 35.1 per cent for Republicans, according to the US Elections Project at the University of Florida.

In 2022, that gap widened, with Democrats accounting for 42.5 per cent of early ballots and Republicans 33.8 per cent.

-with AAP